Absorption Cooling: A Review of Lithium Bromide-Water Chiller Technologies

Recent Patents on Mechanical Engineering 2009, 2, 193-213

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Absorption Cooling: A Review of Lithium Bromide-Water Chiller Technologies Xiaolin Wang and Hui T. Chua* Western Australia Geothermal Centre of Excellence, School of Mechanical Engineering M050, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Received: May 20, 2009; Accepted: July 7, 2009; Revised: July 17, 2009

Abstract: Lithium bromide-water absorption cooling technologies is underpinned by the continual circulation of water, which is the refrigerant, powered by the hygroscopic nature of aqueous lithium bromide solution. The present energy and global warming crises engender a renewed interest in thermally driven cooling systems, for which lithium bromide-water absorption chiller is an archetypal example forming the focus of this review. We review more than 100 patents dated mainly from 1985 and later that offer panoply of solutions to better this genre of timely technologies. We classify the surveyed patents into four main categories: (1) absorption system developments, (2) machine components developments, (3) working fluid modifications and additives, and (4) novel applications of absorption cooling systems. Finally we also map out several important directions for future research and development.

Keywords: Lithium bromide-water absorption chiller, absorption cooling, hygroscopic & global warming. 1. INTRODUCTION The energy and global warming crises have drawn renewed interests to thermally driven cooling systems from the air conditioning and process cooling fraternities. The lithium bromide-water absorption chiller is one of the favourites due to the following specific reasons: (i) it can be thermally driven by gas, solar energy, and geothermal energy as well as waste heat, which help to substantially reduce carbon dioxide emission; (ii) its use of water as a refrigerant; (iii) it is quiet, durable and cheap to maintain, being nearly void of high speed moving parts; (iv) its vacuumed operation renders it amenable to scale up applications. LiBr-H2O absorption chillers enjoy cooling capacities ranging from kilowatts (kW) to megawatts (mW) which match with small residential to large scale commercial or even industrial cooling needs. However they currently enjoy only a fraction of the extent of deployment as their vapour compression counterparts. Their major debilitating factors are a low Coefficient of Performance (COP), larger footprint and required headroom, corrosion and crystallization issues and stringent requirements of vacuum leak tightness over its design lifespan. Over the past 30 years, extensive efforts have been devoted to: (i) develop advanced absorption cycles which could work at low heat source temperature or recover more heat to improve system performance; (ii) improve the design of major components such as generator and absorber to enhance their heat and mass transfer efficacy; (iii) avoid crystallization problem; and (iv) develop new and reliable working pairs. Published review papers are

*Address Correspondence to this author at the WA Geothermal Centre of Excellence, School of Mechanical Engineering M050, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Tel.: 61-8-64881828: Fax: 61-8-64881024; E-mail: [email protected] 1874-477X/09 $100.00+.00

available to summarize these research efforts and achievements [1-3], but are limited to research papers hailing mainly from academia. Consequently, we focus on a comprehensive state of the art patent review with information stemming chiefly from the industry. 2. ABSORPTION COOLING PROCESS The internal operation of a lithium bromide-water absorption chiller is intimately influenced by the pressures and concentrations of its working fluid. In its most basic form, there are four intrinsic components to a lithium bromide-water absorption chiller: an evaporator, a generator, an absorber and a condenser. In the generator refrigerant vapour is thermally desorbed from the solution, which is then condensed in the condenser. The liquid refrigerant in the condenser is throttled and sent to the evaporator where cooling is provided. In the evaporator, the liquid refrigerant vaporizes and the vapour is absorbed by the solution which is actively cooled in the absorber. The refrigerant rich solution is pumped to the generator for generation, while the resultant refrigerant-weak solution is throttled back to the absorber to absorb the refrigerant vapour coming from the evaporator. To improve the system efficiency, a solution heat exchanger is introduced into the solution circuit to recover the energy of the refrigerant-weak solution when it is throttled from the generator to the absorber. The performance index of an absorption chiller is termed the Coefficient Of Performance (COP) and is generally defined as the ratio of cooling output at the evaporator to the heat input to the generator. The first absorption chiller was developed by Edmond Carré in 1850 using water and sulphuric acid. His brother Ferdinand Carré patented a commercial ammonia-water refrigerator in 1873 [4]. However absorption chillers have only started to enjoy serious deployment since the late 1960s when the standard single-effect LiBr-H2O absorption cycle © 2009 Bentham Science Publishers Ltd.

194 Recent Patents on Mechanical Engineering 2009, Vol. 2, No. 3

was used. In 1970, Trane introduced the first mass produced double-effect LiBr-H2O absorption chiller. Ever since then LiBr-H2O absorption chillers became an effective means to harness thermal energy. 3. SYSTEM PROCESS DEVELOPMENT Novel absorption systems have been invented with different focuses in the past 30 years. For the standard single effect system, simple structure and low cost are pursued. The single-effect double-lift absorption system are proposed and developed for the utilization of low temperature heat sources. However, multi-effect absorption systems are suggested to provide higher efficiency with a high temperature heat source. In order to increase the system performance and avoid crystallization problem, various heat and mass recovery systems, modifications to the generator and the absorber, different working pairs and additives have been developed. Furthermore hybridization of absorption chiller cycle with other cooling cycle(s) promises a higher overall performance as compared with that of each single constituent cycle. 3.1. Single-Effect Absorption Chillers A basic single-effect absorption chiller only consists of an evaporator, an absorber, a generator, and a condenser. Its inherent simplicity results in a low initial cost, small unit size, and high reliability, thereby resulting in a lower maintenance cost. It is suitable for applications with small cooling capacities and where only low temperature heat sources are available. The basic cycle of a single effect absorption chiller is shown in Fig. (1). The early generations

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of absorption chillers were bulky and suffered from low COPs, which hindered the application of such chillers in air conditioning. Uchida [5] from Hitachi disclosed a modular cascaded absorption chiller system comprising a number of chiller units connected to one another, in which chilled water flows through the chiller units in series while cooling water flows through the chiller units in parallel. The directions of flow of the chilled water and the cooling water are reversed to each other in each of the chiller units. In the absorber of each of the chiller units, absorption solution is sprayed in one or more stages. This invention is mainly for a large-tonnage application suitable for district cooling. The chilled water temperature difference in this invention could be 1.4 times larger than that in conventional absorption machine which possibly reduces the amount of chilled water to 70% of that in a conventional machine. It also reduces the amount of cooling water to 75% of that in a conventional machine by exploiting a larger cooling water temperature difference. Furthermore, since the average chilled water temperature is higher, and the average cooling water temperature is lower, the efficiency of this chiller module is better than that in a conventional machine. Consequently, not only does the scheme reduce the size of a chiller system, but also reduces the required capacity of transportation pump, sizes of pipes and cooling tower. In order to promote the application of absorption chillers in residential air conditioning, Inoue et al. [6] from Ebara Corporation integrated the absorber, evaporator, generator and condenser into a single compact housing so that the absorber is located in the lower portion of the housing, the

Fig. (1). A schematic of a single effect absorption cycle in a Dühring plot.

Lithium Bromide-Water Chiller Technologies

evaporator beside and above the absorber, the condenser above the absorber, and the generator above the condenser. Even the refrigerant gas passage extending from the evaporator to the absorber and refrigerant gas passage extending from the generator to the condenser are packed within the housing. This invention eliminates conventional one-housing drawbacks such as large dimension and thermal stress and reduces the cost of absorption chillers due to the reduction of chiller size and material used. Figure 2 shows the schematic of this compact design.

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strong solution immediately. The heated refrigerant-strong solution will then warm the crystallized solution on the opposite side of the solution heat exchanger, thereby dissolving the lithium bromide crystals into the solution and allowing continued operation. This popular technology is applied to both single and double effect absorption chillers. 3.2. Single Effect Double Lift In the commercial market, the hot water temperature for most of single effect absorption chillers is above 90°C. For a given chiller, when the heat source temperature is less than 90°C, its performance will drop significantly. This limits the use of single effect cycles in waste heat applications where the heat source temperature is normally between 70°C to 90°C, while waste heat of higher temperatures is often found to have alternative usages. To address this problem, a single effect double lift cycle was developed, and this advanced cycle can utilize hot water temperatures down to 55°C with its COP varying between 0.35 and 0.7 [9]. The single effect double lift cycle, as shown in Fig. (3), is the integration of a double lift cycle into a single effect cycle. The evaporator, absorber 1, generator 1, and the condenser forms the single effect cycle, while the evaporator, absorber 1, generator 3, absorber 2, generator 2, and condenser constitute a double lift cycle. When heat is supplied to a single effect double lift system, higher temperature heat can be used to power the single effect cycle, while the lower temperature heat can be used to power the double lift cycle. Depending on the proportion of heat supplied to the two cycles, the COP of this integrated single effect double lift cycle varies between 0.35 and 0.7.

A - Absorber, C - Condenser, E - Evaporator, G - Generator, X Solution heat exchanger, SP - Solution pump, RP - Refrigerant pump Fig. (2). A single effect absorption chiller [adapted from ref. 6].

In a further bid to reduce the system size and improve the system COP, Inoue et al. [7] further disclosed an absorption chiller machine in which plate type heat exchangers are used in the absorber and condenser. In this invention, the cooling water flows into the absorber and condenser in parallel and is distributed according to the fluid resistance of each of the plate type heat exchangers. This arrangement substantially reduces the flow rate into the absorber and condenser and in turn reduces the size of the plate type heat exchanger. The water distribution mainly depends on fluid resistance which eliminates the complicated flow regulating valve system. The detailed configuration will be discussed in a later section, viz. system component development. Motivated to avoid crystallization, the J-tube technology has been developed and widely applied [8]. When incipient crystallization occurs, it would typically begin in the refrigerant-weak solution side of the solution heat exchanger. This forces the refrigerant-weak solution to accumulate in the generator. When the solution reaches a certain level in the generator, the hot refrigerant-weak solution will overflow via the ‘J’ tube to the absorber and warms the refrigerant-

In the operation of a single effect double lift cycle, refrigerant vapour is first absorbed in absorber 1, the refrigerant rich solution is first sent to generator 1 to be generated, then serially to generator 3 to be further generated, and finally the refrigerant-weak solution is returned to absorber 1. The refrigerant vapour emanating from generator 1 is sent to the condenser to be condensed, while the refrigerant vapour stemming from generator 3 is absorbed by absorber 2, and the resultant solution is sent to generator 2 for generation. The hot water first flows into the generator 1 to heat up and boil the refrigerant-rich solution and then serially flows into generator 2 and then generator 3 to generate their respective solutions. The cooling water flows into the condenser, absorber 1 and absorber 2 in parallel in order to avoid a complicated control system and unstable working conditions. The heat supplied to the single effect double lift scheme is more efficiently utilized than the single effect scheme. Hence for the same quality of energy input, the single effect double lift system will regenerate more refrigerant vapour than the single effect system, which implies more refrigerant will be supplied to the evaporator, and the cooling energy provided will be increased. The increased number of components means however that the size of a single effect double lift chiller is much bigger than the single effect machine. This type of absorption chiller has already been commercialized by INVEN Absorption GmbH [10].


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Fig. (3). A single-effect double lift cycle.

3.3. Double Effect Absorption Chiller Due to the relatively low COP associated with the single effect technology, it is difficult for single effect machines to compete economically with conventional vapour compression systems except in low temperature waste heat applications where the input energy is virtually free. It is always desirable to increase the efficiency of an absorption chiller, to increase its compactness and to decrease its operating costs. Double effect absorption chiller technology enjoys a higher COP, in the range of 1.1 to 1.3. It was first patented by Loweth [11] in 1970 and commercialized by Trane in the same year. Saito [12] from Ebara Corporation and Alefeld [13] improved and modified the double effect absorption refrigeration machine in 1980 and 1985, respectively. The major difference between the single effect and double effect absorption cycle is that the latter incorporates a high temperature generator operating at a higher temperature and pressure in addition to the standard components of a single effect system, resulting in a more efficient utilization of the supplied heat. Figure 4 shows the schematic of a double effect absorption cycle in the Dühring plot format. Heat from the high temperature source is transferred into the high temperature generator and heat from the user is transferred into the evaporator. Heat is transferred out from the cycle in the condenser and absorber. The cycle includes two regenerative solution heat exchangers which have a similar role in the solution circuits as that in the single effect

cycle. The critical feature of a double effect cycle is the internal heat exchange associated in the low temperature generator in which the high temperature refrigerant generated in the high temperature generator exchanges the heat to further generate the solution. The low temperature generator and condenser effectively function as part of a single effect cycle. The typical operating temperatures and pressures of the high pressure end of a double effect scheme can be inferred from Fig. (4). The heat input to the double effect cycle occurs at a much higher temperature than in the single effect cycle; however heat rejection at the condenser and cooling at the evaporator are at about the same temperatures as a single effect cycle, resulting in a higher COP. The double effect absorption cycle, as shown in Fig. (4), has been commercialized by most absorption chiller companies since 1970. The flow arrangement of the working solution into the high and low temperature generators is serial. The refrigerant-rich solution flows into the high temperature generator first and is generated by the external heat source, this generated solution is then introduced into the low temperature generator and further generated by the refrigerant vapour from the high temperature generator. This is a standard flow scheme commonly used by almost all suppliers. However in this arrangement, the operating condition is very close to the crystallization line of LiBr-H2O solution and the high temperature generator operates typically at a high pressure in order to maintain the requisite



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Fig. (4). A double effect series flow type absorption cycle.

solution flow rate. In a further bid to avoid solution flow pumping, the high temperature generator has to be sufficiently elevated to enjoy gravity assisted flow, resulting in the need for high headroom. In order to avoid these problems, Hitachi [14] developed a double effect parallel flow cycle as shown in Fig. (5). In this flow scheme, the refrigerant-rich solution produced in the absorber is separated after the solution pump outlet and sent simultaneously to the high and low temperature generators through the high temperature solution heat exchanger and low temperature solution exchanger, respectively. From Fig. (5), it is evident that the operating condition of this parallel flow scheme is further away from the crystallization line of LiBr-H2O solution in comparison to the series flow scheme. And since the solution flow rate to the high temperature generator is significantly lower, the pressure within the generator and its elevation are also significantly reduced. Xu [15, 16] theoretically analysed the performance of these two types of double effect cycles and the influence of each flow type on the design parameters of the heat exchangers like solution circulation ratio, heat-recovery ratio, and distribution ratio on the performance of the system. Chua et al. [17] compared their relative performance and concluded that the parallel flow scheme resulting in a slightly higher COP than the serial flow scheme. These two major types of double stage machine are presently favourites for trigeneration applications, where power, heating and cooling are supplied.

Nagao [18] from Hitachi disclosed an absorption chiller that consists of an evaporator section, an absorber section, a condenser section and a generator section, all of which are divided into two stages. The first stage evaporator and second stage evaporator are arranged to be enclosed respectively by the first stage absorber and second stage absorber, thereby forming two separate units each containing an evaporator and absorber. Similar configuration also applies to the generators and condensers thereby forming two separate units. Such a system arrangement substantially eliminates the heat dispersion of the system and improves the overall efficiency. Motivated to improve the system efficiency and avoid stagnation when passing refrigerant from the condenser to evaporator during chiller start up, Hiro [19] from Sanyo Electric Co. disclosed a double effect absorption chiller, in which the connecting pipe conveying the liquefied refrigerant in the low temperature generator to the condenser is installed with an orifice together with a control valve so as to control the refrigerant pressure. A control circuit is connected to this control valve to actively control the refrigerant pressure and therefore control the solution concentration in the high temperature generator and the absorber. This control circuit facilitates the passage of the refrigerant to the condenser without stagnation during chiller start up or in the event of a sudden increase in cooling load. It is also capable of maintaining a suitably reduced pressure in the refrigerant during steady-state operation so as to achieve a higher operating efficiency.


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Fig. (5). A double effect parallel flow type cycle.

More recently, Aoyama et al. [20] from Ebara Refrigeration Equipment & System Corporation disclosed an internal heat recovery scheme in conjunction with a heat scavenging scheme for a double effect absorption chiller. The internal heat recovery scheme aims to improve the intrinsic COP of the machine, while the heat scavenging scheme strives to extract more energy from the heat source which drives the chiller. The latter is particularly attractive if the heat source is a waste heat source. According to this invention, the refrigerant rich solution path leading from the absorber to the high temperature generator is divided into two routes. The first route is installed with one or two drain heat exchanger to scavenge the remaining enthalpy of the heat source powering the high temperature generator. The second route is installed with one or two regenerative heat exchanger to recover the heat of the hot and refrigerant-weak solution leaving the high temperature generator. All the heat exchangers adopt the counter flow scheme to maximize heat transfer efficiency. Therefore the overall system efficiency is potentially higher than a standard double effect absorption chiller.

Nevertheless in practice the COP of single effect cycles is on the order of 0.8, creeping higher only in part load operation. In order to overcome this limitation, numerous multi-effect or multi stage cycles have been proposed.

3.4. Multi Effect Absorption Cycles

In large tonnage application and when very high temperature (i.e. significantly higher than 170°C) heat is available, double effect chillers will be an inefficient choice from the thermodynamics standpoint. Significantly higher COP can be achieved by adopting triple effect chillers. Such chillers enjoy a COP of 1.6 and higher effect cycles will have COP greater than 1.6. The same principle that extends the single effect to double effect is used to form triple effect

This section specifically addresses multi effect cycles, other than double effect cycles. Heightening the COP of absorption cycles has always been a holy grail. Many standard and well-known measures are routinely applied to single effect absorption cycles to maximize their COP, such as sensible heat recoveries in their various forms.

3.4.1. One and a Half Effect Cycle A one and a half effect cycle is disclosed by Erickson [21] which reportedly achieves a COP of approximately one and a half time that of a single effect cycle, at generator temperatures substantially less than the minimum required for double effect operation. This is done by providing two internally heated generators in addition to an externallyheated generator; plus at least two disparate absorbergenerator solution circulating loops; and a transfer of vapour from one of the generators to one of the absorbers. Another key aspect of this invention is that the disparate absorbergenerators circulation loops allow for the use of completely different absorbents in the respective loops provided they pair with the same working fluid. The schematic of such an apparatus and process is shown in Fig. (6). 3.4.2. Triple Effect Cycle



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A - Absorber, G - Generator, E - Evaporator, C - Condenser Fig. (6). A one and a half effect cycle [adapted from ref. 21].

1 - Low temperature generator, 2 - Medium temperature generator, 3 - High temperature generator Fig. (7). A triple effect cycle by Oouchi et al. from Hitachi [adapted from ref. 22].

system from double effect system. Oouchi et al. [22] from Hitachi firstly invented a triple-stage absorption refrigeration system that has three generators, viz. the high temperature generator, the medium temperature generator and the low temperature generator, and the working solution is supplied from the absorber directly to each of the three generators in parallel. The cooling medium is made to flow firstly through the condenser and then through the absorbers. This parallel solution flow scheme helps to decrease the temperature and pressure of the highest temperature generator, which in turn renders the vessel wall corrosion problem more manageable. Figure 7 shows the configuration of such a triple effect cycle. DeVault et al. [23] noted that the refrigerant vapour emanating from the high temperature generator, having shed its latent heat to power the medium temperature generator, turns into hot condensate. They used the high temperature sensible heat of this condensate, in addition to the latent heat of the refrigerant vapour emanating from the medium temperature generator, to power the low temperature

generator, resulting in heightened efficiency. They reported that with the high temperature generator at 228°C, a parallel flow scheme gives rise to a COP of 1.68, whereas a serial flow scheme results in a COP of 1.38. DeVault et al. [24] proposed a variant of the triple effect absorption chiller involving two single effect absorption circuits, each working at a different temperature range. The high temperature circuit receives the primary heat input, and its condenser and absorber are designed to reject heat at a high temperature. The resultant heat rejection from this high temperature circuit is used to fire the generator of the low temperature circuit, reportedly generating about twice the amount of refrigerant as its high temperature counterpart. Effectively the primary heat input is used three times in terms of refrigerant generation and hence the triple effect cycle. The evaporators of both circuits operate at about the same temperature so that both evaporators supply useful cooling to the end user. They reported that the design was


30% to 50% more efficient than a conventional double-effect absorption chiller. Rockenfeller et al. [25] eliminated local hot spots in the high temperature generator in a triple effect chiller by introducing a heat transfer loop between the gas flame and the generator heat exchanger, so as to achieve more homogenous heating. This heat transfer loop also scavenges the enthalpy of the effluent gas to preheat inlet air for combustion, or to assist with the heating of the medium and low temperature generators. They also considered the arrangement of several evaporators and, in particular, several absorbers so as to match the operating temperatures and the solution property. The multiple absorbers at different temperatures relax the constraint of returning the hot solution from the high temperature generator to a single low temperature absorber, thereby improving the robustness against solution crystallization. They further identified several lithium corrosion inhibitors such as lithium molybdate, lithium nitrate or lithium chromate, and proposed lithium hydroxide for pH adjustment. Heat and mass transfer additives were also considered, such as 2-ethylhexanol and n-octanol. They proposed several schemes to prevent the additive from entering the high temperature generator, as it was generally unstable at high temperature. In order to reduce the temperature and pressure of high temperature generator and mitigate corrosion, Lee et al. [26] disclosed a triple effect absorption chiller with vapour compression units where one or more compressors are connected to the high temperature generator, medium temperature generator or evaporator of a conventional chiller. The vapour compression units compress refrigerant vapour from the generators or evaporator, thereby lowering the high generator temperature to a preferable range of below 170°C and raising the pressure of the absorber. The triple effect absorption chiller of this invention can be practically applied to lower the temperature of the high temperature generator and to mitigate corrosion. More recently, Inoue [27] from Ebara Corporation introduced an auxiliary absorber and auxiliary low temperature generator to the standard triple effect cycle so as to control the high temperature generator temperature and pressure, particularly during periods of heightened cooling water temperatures, and to maintain good operating efficiency. The strategy also allows the use of common stainless steel material for the high temperature generator, thereby facilitating commercialization. In typical operation, both the auxiliary and the standard low temperature generators serve to concentrate the solution, which is then sent to the absorber. The refrigerant-rich solution from the absorber is then sent in parallel to the auxiliary, medium temperature and high temperature generators. Steam from the medium temperature generator is sent in parallel to power both the low temperature and auxiliary generators. The principal difference is that the steam from the auxiliary generator is absorbed by the auxiliary absorber. All the resultant dilute solution is then fed into the low temperature generator, effectively lowering its temperature, which then has a cascaded effect on the temperatures of the medium and high temperature generators. A reduction of 20°C in the high temperature generator has been reportedly achieved. With rising cooling

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water temperature, more solution from the absorber is channelled to the auxiliary absorber to be further diluted and then sent to the low temperature generator, followed by the auxiliary generator for further generation before returning to the absorber, thereby relying more on the low temperature operation, instead of the high temperature operations. In addition, Perez-Blanco [28] proposed a conceptual mass and heat exchange cycle. This concept reduces irreversibilities further by allowing mass exchange in addition to heat exchange among stages. Hence the heightened efficiency of triple effect chillers is accessible with doubleeffect temperatures. 3.4.3. Seven Effect Cycle DeVault et al. [29] disclosed a seven-effect absorption refrigeration system that utilizes three absorption circuits and has a similar working principle as their previous triple-effect absorption chiller [23, 24]. They claimed a COP in the range of 2.19 to 3.12. Sodium hydroxide-water solution pair, where water is the refrigerant, is used for the high and medium temperature absorption circuits in this invention. Apart from the abovementioned patents, there are some other patents such as US3742728 [30], US3831397 [31], US4442677 [32], US4551991 [33] and US4531374 [34] which disclosed variable effect absorption chillers. They will not be discussed here due to page limitation and their antiquity. 3.5. Air-Cooled Absorption System Most commercial absorption chillers are water cooled so as to enjoy high COP and compactness of design, along with low electricity consumption. However, with an ever stricter regulation on the water quality and on-going monitoring, air cooled chillers are being developed. A nagging problem for air-cooled LiBr-water absorption chiller is crystallization. An air-cooled absorber will typically be warmer than a watercooled one, and therefore will require a higher concentration of LiBr to achieve the required and lower vapour pressure, thereby making it more susceptible to crystallization. Leonard [35] was the first to invent an air cooling technology for the absorption chiller. But it demands the use of fluorocarbons as the refrigerant to heighten the operating pressure and temperature in both the absorber and condenser. This idea remained dormant until Kunugi et al. [36] from Hitachi disclosed a parallel type, double effect air cooled absorption chiller in which lithium bromide-water system is used Fig. (8). The absorbing solution is sprayed and allowed to flow downward in a finned tube array, while the cooling air across the heat exchanger is made to flow upward, thereby achieving a counterflow scheme. They also considered the beneficial effect of having a second downstream counterflow finned tube absorber where the dilute solution is further cooled by the vapour refrigerant leaving the evaporator to enhance absorption. Later Kunugi et al. [37] realized that the counterflow mass transfer between the vapour refrigerant and the absorbing solution was a bottleneck. They improved the design and allowed the vapour to enter the tubular absorber from the top and bottom, and even from midway to enhance mass transfer.



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1 - High temperature generator, 2 - Low temperature generator, 3 - Absorber, 4 - Evaporator, 5 - Condenser Fig. (8). An air cooled absorption chiller [adapted from ref. 36].

101 - High temperature generator, 102 - Low temperature generator, 103 - Air cooled condenser, 104 - Evaporator, 105 - Air cooled absorber, 106 - High temperature solution heat exchanger, 107 - Low temperature solution heat exchanger, 108 - Refrigerant pump, 109 Solution pump, 110a/d - Solution spray pump, 111 - Pre-cooler, 112/113 - Air extracting apparatus, 114 - Liquid vapour separator, 115 - Air tank, 116 - U-shape sealing tube, 117 - Solution tank, 124 - Refrigerant vapour conduit, 125 - Refrigerant vapour conduit, 127 - Refrigerant conduit, 131 - Three-way changeover valve, 132/134 - Changeover valve, 137 - Three-way changeover valve, 151-152 - Pressure reducing valve, 154 - Refrigerant spray nozzle, 156/157 - Air extracting tube. Fig. (9). A parallel type double effect absorption cooling and heating system [38].

As shown in Fig. (9), Ohuchi et al. [38] from Hitachi disclosed a parallel type double effect absorption cooling and heating system, in which the air cooled condenser and absorber has a cross flow configuration between the solution/ refrigerant and the cooling air. They endeavoured to optimize the temperature match between the solution/ refrigerant and the cooling air, so that the hot condensate emanating from the low temperature generator is cooled in the last row of the air-to-solution/refrigerant heat exchanger, while the vapour refrigerant emanating from the low temperature generator is cooled in the first row. The solution cascades from the second last row progressively to the second row of the heat exchanger via the connecting pipes between the dedicated solution pumps, absorbing refrigerant from the evaporator from both the top and bottom of the

tubular heat exchanger to enhance mass transfer. The same connecting pipes also ensure hydraulic stability. The solution flow rate in the heat exchanger is larger than that serving the generators, so as to eliminate dry regions within the heat exchanger and maximize heat and mass transfer. By the judicious use of three way valves, the machine can be configured into a vacuumed boiler, in which the low temperature generator, absorber and condenser are isolated to minimize heat loss, while the vapour refrigerant generated by the high temperature generator is condensed in the evaporator, thereby generating heating, and the condensate is sent straight back to the high temperature generator. Kurosawa et al. [39] from Hitachi, Osaka gas Corporation and Toho gas Corporation compacted Oouchi et al. invention [38] by positioning the cooling fans on the side


panel of the chiller assembly, instead of disposing them vertically on the top panel. The top panel is covered with an inclined roof, so as to prevent snow accumulation and icicles from entering the interior of the machine, which ultimately might damage the fans. Referring to Fig. (10), they improved the machine’s purging of non-condensible gas and its robustness against over-condensation of the absorption liquid. Solution liquid venturi effect is exploited to purge non-condensible gas from the lower header of the condenser. Due to the high pressure within the condenser, the venturi extraction can be achieved with the pumping of dilute solution in the first row of the absorber. The conco-mitant absorption of vapour refrigerant in the condenser sends the non-condensible gas to the gas storage tank via the gas-liquid separator. The active pumping effect ensures that the gas storage tank is at a higher pressure than the condenser, allowing for easy purging from the chiller system. The gasliquid separator is pressure sealed from the returning line to the generators via an inverted U-bend. The non-condensible gas from the lower header of the absorber is purged similarly from its coolest end, except that its low pressure mandates the use of concentrated solution in the last row of the absorber, coupled with active air cooling upstream of the condenser after the venturi extractor, which is located higher than that for the condenser to maintain a hydrostatic seal. Kurosawa et al. also installed a gas and liquid mixture flowdown pipe between the lower header of the first-row absorber tube and the weak solution tank, directly above the solution flow pump servicing the generators, so that noncondensible gas can be directly discharged into the vapour chambers of the generators. When the liquid refrigerant is overly condensed, excessive liquid refrigerant overflows and

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merges with the dilute solution in the first row of the absorber, thereby further diluting the solution and preventing crystallization in the low tempe-rature heat exchanger. The well primed piping leading to the solution pump offers a liquid seal against the parasitic suction of non-condensable gas, ensuring the integrity of the gas purging mechanism. The overflowed liquid refrigerant also cools the dilute solution, heightening the downstream vapour refrigerant absorption, thereby densifying the non-condensible gas, further assisting with its purging. Gonzalez-Cruz et al. [40] invented a compact solar air conditioning system which utilized a single effect lithium bromide-water absorption chiller. They eliminated the need for a cooling tower and manufactured the air-cooled condenser and absorber with copper pipes and aluminium fins so as to reduce the initial cost. Fans are separately attached to the condenser and absorber to provide active cooling to the components. Ohuchi et al. [41] from Hitachi invented an air cooled parallel type double effect absorption chiller with a compact absorber. Referring to Fig. (11), they installed an integral generation chamber and absorption chamber between the absorber and generators. Part of the dilute solution from the absorber is generated in the generation chamber powered by concentrated solution returning from the two generators. The resultant concentrated solution is returned together with the concentrated solution from the generators to the absorber, thereby substantially increasing the solution flow rate within the absorber. The vapour refrigerant from the generation chamber is absorbed by the balance dilute solution from the absorber, further diluting and heating up the dilute solution

11 - High temperature generator, 12 - Low temperature generator, 13 - Condenser, 14 - Evaporator, 15 - Absorber, 16/36 - Hot water heat exchanger, 17 - High temperature heat exchanger, 18 - Low temperature heat exchanger, 19 - Solution pump, 20 - Refrigerant pump, 21/48 Pre-cooler, 22 - Automatic bleeder, 23 - Weak solution tank, 24/47/51 - Gas-liquid flow down pipe, 25 -Vertical tube, 26 - Lower header, 27 - Liquid refrigerant tube, 28 - Throttle, 29-32 - Changeover valve, 33-34 - Throttle, 35 - Heat transfer tube, 40 - Gas storage tank, 41 - Gas liquid separator, 42 - Gas ascension pipe, 43 - U-shape seal tube, 44/50 - Bleeder pipe, 45/49 - Gas mixing chamber, 46 - Solution pipe, 52 Overflow pipe, 53 - Liquid refrigerant heat exchanger, 61a-d - Vertical tubes, 62a-d - Header, 63a-d - Spray pump, 64a-d - Spray nozzle, 65a-d - Suction pipe, 66a-d - Connection pipe. Fig. (10). A schematic of a double effect absorption cooling and heating system [39].



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1 - High temperature generator, 2 - Low temperature generator, 3 - Condenser, 4 - Evaporator, 7 - High temperature heat exchanger, 9/14/19 - Solution pump, 10 - Refrigerant pump, 11 - Chilled water, 12 - Medium-temperature heat exchanger, 13 - Hot water, 15 - Integral generation chamber, 16 - Integral absorption chamber, 17 - Solution heat exchanger, 18 - Heat exchanger, 20 - Spreading matrix, 21/24 Pressure reducing valve, 40 - Air-cooling fan, 41 - Absorber. Fig. (11). A schematic of an air cooled parallel type double effect absorption chiller [41].

before being channelled to the generators. The generators thereby enjoy a larger solution concentration difference, resulting in a reduction of solution flow rate for the same amount of refrigerant flow to the evaporator, and thus heightening the COP. On the other hand, the increased solution flow rate in the absorber allows for a reduction in the heat transfer area as well as cooling air flow rate, with the latter resulting in lower electrical power consumption and noise level. In the air cooled absorption cycles, there is a type in which a thermosiphon is used to cool the absorber or heat the generator. The first two patents were disclosed by Willem et al. [42, 43] from US Philips Corporation in which external heat is delivered from a heat source to the generator via thermosiphon. 3.6. Absorption Chillers with Heating and Cooling Features Combined cooling and heating absorption systems had been disclosed since 1958 [44] and modified and improved over a few decades. There are three major types to this genre: one type simultaneously produces cold and hot waters without effecting a changeover of the refrigerant circuit [4447]; another type performs a cooling operation with a direct expansion system between air and a refrigerant, and a heating operation by producing hot water using the heats of adsorption and condensation [48-50]; the third type relies on the changeover of the refrigerant circuit to produce either cold or hot water [51, 52].

Inoue et al. [53] from Ebara Corporation disclosed a double effect absorption cold or hot water generating machine that has a bypass circuit for directing refrigerant liquid from the evaporator directly to a diluted solution circulating system during a warming operation, in which a high head canned pump is disposed in the solution path between the absorber and the high temperature generator and a low head pump between the absorber and low temperature generator. Both the high and low head pumps are driven during a cooling operation and only the high head pump is driven during a warming operation, resulting in a reduction of electricity consumption. In addition, only the canned motor of the high head pump needs to be heat resistant, whereas the low head pump can be of a cheaper grade. Inoue et al. [54] later invented a double effect absorption simultaneous cooling and heating machine. They introduced a water heater powered by the condensation of vapour refrigerant from the high temperature generator, and the condensate is mostly returned to the evaporator. Heating part load performance is achieved by controlling the extent of flooding of the water heater. When the water heater is fully flooded, further part load performance is realized by channelling some of the cooling air or water through a heat exchanger in the hot water return line, thereby ensuring good part load temperature control. Tongu [55] from Yazaki Corporation disclosed a double effect air-cooled absorption cooling and heating system which avoids the use of separate heat exchangers, and an expensive large bore three way valve for cooling and heating operations. They employed a coaxial tube heat exchanger within the evaporator chamber. In a cooling operation,


chilled water flows in the annular cross section of the heat exchanger, while refrigerant condensate evaporates from the outer surface of the heat exchanger into the evaporator chamber; fluid within the inner tube of the heat exchanger remains stagnant. During a heating operation, vapour refrigerant from the high temperature generator condenses within the inner tube of the heat exchanger and returns to the high temperature generator, thereby heating the hot water flowing in the annular cross section of the heat exchanger. Refrigerant is prevented from entering the evaporator chamber and the rarefied environment stifles heat loss from the outer surface of the heat exchanger. Koseki et al. [56] from Hitachi focused on the load responsiveness of an absorption cooling and heating machine in the context of central air conditioning. They instituted multiple gas burners, variable speed solution pump, variable speed refrigerant spray pump in the evaporator and variable cooling water flow control to enhance load matching and preserve COP at part loads. When the operation stops, refrigerant from the condenser and low temperature generator is directly channeled to the absorber to prevent crystallization. The store of refrigerant in the evaporator is rapidly sprayed when the operation restarts for rapid load response. Temperature differences between the absorber and condenser and between the high temperature generator and condenser are measured online to estimate solution concentration and forestall possible crystallization. Signals from the various indoor units are used for predictive control of the central plant to improve load responsiveness. Ohuchi et al. [57, 58] from Hitachi improved on the operation friendliness of a double effect absorption central air conditioning plant. The zone controllers in a standard variable air volume system are linked to the control unit of the absorption machine. During cooling operation, when any one zone controller receives a stop signal, the air handling unit (AHU) for that zone is stopped instantly and the control unit stops fuel supply momentarily to prevent a rapid reduction in the chilled water temperature. When the zone controllers receive stop signals in sequence, a safety operation might be activated where the fuel input and the solution supply to the absorber are stopped or liquid refrigerant from the evaporator is sent to the absorber for rapid dilution. When the entire cooling operation is stopped, liquid refrigerant is sent from the evaporator to the absorber and the solution flow rate to the absorber is increased for rapid dilution. The dilution procedure is subsequently terminated and the plant shut down in sequence. During the automated start-up phase, when the cooling water temperature is low, the solution flow rate to the high temperature generator is increased. The heating operation of the plant is also similarly automated, allowing for operator friendliness and improved safety. 3.7. Hybrid Systems An absorption refrigeration system can be integrated with a vapour compressor unit to form a hybrid system. Shiflett and Yokozeki [59, 60] proposed that when the electricity tariff is high, the system could rely more on the absorption portion of the unit, and when the tariff is low, the reliance could shift to the vapour compression portion of the unit

Wang and Chua

instead with improved load responsiveness. In their hybrid unit, a portion of the refrigerant vapour produced at the evaporator is sent to the vapour compressor then to the condenser, and the balance is absorbed in the absorber. They had also identified compatible refrigerants and absorbents such as fluorocarbon gases in fluorinated ionic liquids. Kim and Lee [26] disclosed a vapour compression assisted triple-effect absorption chiller in which one or more vapour compressor pressurizes the vapour from the high temperature generator, middle temperature generator or evaporator, thereby lowering the high temperature generator temperature to below 170oC and mitigating the corresponding corrosion problem. Meckler invented a hybrid double effect chiller which comprises an additional regenerative vapour recompression absorber (VRA) as shown in Fig. (12) [61]. He aimed to further concentrate the solution to the absorber thereby realizing a large temperature difference to the chilled water and in turn reducing chilled water pumping and piping costs. A portion of the solution from the high temperature generator is sent to the upper chamber of the VRA in which it is diluted by vapour sucked from the lower chamber with the help of a blower and returns to the high temperature generator. The balance of the solution from the high temperature generator, together with solution from the low temperature generator, are further generated at the lower chamber using the heat of absorption from the upper chamber before being sent to the absorber. The heat exchanger material of the VRA is polymeric to mitigate fouling and reduce cost. Kim and Lee [62] invented the hybrid use of hightemperature steam and medium-temperature water generated in industrial processes as heat sources for an absorption chiller. In addition to a standard double effect chiller, they introduced an additional generator powered by the medium temperature water which receives dilute solution from the absorber in parallel to the solution stream sent to the standard high temperature and low temperature generators. 4. SYSTEM COMPONENTS DEVELOPMENT 4.1. Absorber, Evaporator and Condenser Hiro et al. [63] from Sanyo Electric Corporation disclosed an absorber heat exchanger as shown in Fig. (13) that consistes of an array of vertical flat plates spaced apart from one another with cooling water pipes extended through those plates. The solution trickles through the holes from the Vshaped receptacles, thereby coalescing into thin films on the plate and increasing its contact area and contact time with both the plates and the vapour refrigerant. Inoue et al. [7] from Ebara Corporation employed plate type heat exchangers for both the absorber and condenser. Cooling water services the absorber and condenser in parallel, resulting in better heat transfer performance due to lower flow rate and smaller required passage area. Greater flow rate to the condenser also lowers the condensation temperature, which in turn lowers the temperature of the high temperature generator, resulting in effective corrosion suppression.



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2 - Inlet of solution heat exchanger leading from high temperature generator, 4 - Solution heat exchanger outlet, 5 - Solution heat exchanger inlet, 6 - Outlet of solution heat exchanger to high temperature generator, 8 - Solution inlet, 9 - Outlet leading to absorber, 60 - Absorption chamber, 61 - Evaporation chamber, 62 - Heat transfer surface, 63 - Evaporative surface, 64 - Nozzle, 65/73 - Manifold, 66 - Nozzles, 67 Sump, 68 - Cylindrical tank, 69 - Polymer panels, 70/70 - Top and bottom wall of tank, 71/72 - Top and bottom header, 74/76 - Pump, 75 Heat exchanger, VC - Vapour compressor, M - Motor. Fig. (12). A regenerative vapour recompression absorber (VRA) [61].

Inoue et al. [64] also invented plate type heat exchangers for the evaporator, condenser, absorber and generator of a chiller. The plates have depression structures that are brazed into contact with each other as illustrated by Fig. (14). Lowenstein [65, 66] from Gas Research Institute focused on a small capacity, air cooled, direct expansion absorption chiller. To this end, he disclosed modular plate type heat exchangers for both the evaporator and absorber so that they can directly undergo heat exchange with the cold air and cooling air, respectively. Kuhlenschmidt and Klintworth [67, 68] from Gas Research Institute intensified heat and mass transfer within a water cooled absorber by having a vertically disposed cylindrical heat exchanger housed with a number of concentric cylindrical heat exchanging walls. The coolant and solution flows in alternating annulus channels, with the coolant flowing upward, the solution traversing downward and the vapour refrigerant undergoing counterflow heat and mass transfer with the solution. They further installed helical coils into all the annulus channels to promote heat and mass transfer. The helical coils in those annulus channels in which solution flow was further grooved to promote the formation of falling solution thin films thereby further enhancing heat and mass transfer.

1 - Heat transfer plates, 2 - Cooling water pipe, 5 - Absorber, 10 Absorbent receptacles, 11 -Distributing holes, Td - Thickness of plates, Pd - Pitch of plates. Fig. (13). A plate type absorber design [63].

Furukawa et al. [69] from Sanyo Electric Corporation invented a horizontal heat exchanging tube array for the absorber, condenser and evaporator of an absorption chiller. They engineered protruding spiral threads on the inner surface of the tubes to create turbulence, and spiralling rows of tiny studs on the outer surface to counter surface tension. To further enhance heat transfer, the spiralling direction of the inner protruding threads and the outer studs are made opposite. For the evaporator, they showed that heat transfer


Wang and Chua

Fig. (14). A plate type heat exchanger for various chiller components [64].

coefficient was systematically higher than that for prior art. For the absorber, they showed that 25% improvement over smooth bore heat transfer tube was achievable. 4.2. Solution Heat Exchanger Inoue et al. [70] from Ebara Corporation invented a two pass plate type solution heat exchanger with the unique feature of having a bleeding point in between the two passes. This heat exchanger normally serves to preheat the dilute solution by cooling down the concentrated solution. When however the concentrated solution is overly cooled and there is an incipient risk of crystallization at the outlet port of the concentrated solution, the bleeding point was then activated. This allows the partially heated dilute solution to be channelled in a controlled manner into the partially cooled concentrated solution, thereby forestalling crystallization. As shown in Fig. (15), Fujii et al. [71] from Hitachi invented a falling film heat exchanger that works together with a liquid distributor. The liquid distributor, having a simple configuration, can uniformly distribute the working solution onto the heat exchanger tubes at a consistent feeding and dripping rate even when foreign objects like metal particles are mixed in the liquid. The falling film heat exchanger enjoys high performance, and is scalable.

Fig. (15). A falling film heat exchanger [71].

Conventional heat exchangers normally enjoy large heat exchanger area at the expense of size and weight. Thin plastic film is therefore often considered to be an alternative heat exchanging material. Lowenstein et al. [72] from Gas Research Institute invented a thin plastic-film heat exchanger for an absorption chiller that enjoys improved strength in the turning regions of the fluid passage so as to enhance the resistance to bursting. 4.3. Generator As shown in Fig. (16), Omori et al. [73] from Ebara Corporation disclosed a compact generator which displays the stability of flooded type generators, and the compactness and heat transfer efficacy of the once-through type generators. The generator uses combustion gas as the heat source. The refrigerant-rich solution, having been preheated by the combustion gas, enters the externally finned drum shell, mixes with the pool of refrigerant-weak solution therein and flows down through the bottom opening via the boiling preventing plate into the gap between the circulation guide and the internal wall of the drum shell. The solution is then efficiently generated within the narrow confinement, the resultant liquid-vapor mixture rises and enters the drum shell from the top whereupon the vapour is separated from the liquid and leaves the generator. The presence of the overflow



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1 - Generator, 2 - Drum shell, 3 - Solution inlet of generator, 4 - Solution outlet of generator, 5 - Vapour outlet, 6 - Heat transfer fins, 7 Circulation guide, 8 - Bottom opening, 9 - Top opening, 10 - Overflow weir, 11 - Downwardly facing opening, 12 - Boiling preventing plate, 13 - Burner, 14 - Combustion chamber, 15 - Burner fan, 16 - Combustion chamber cover, 17 - Solution preheater, 18 - Exhaust guide Fig. (16). A compact direct fired generator [73].

weir ensures the retention of a predetermined amount of solution within the drum shell thereby assuring boiling stability. This generator, however, is not amenable to scaling up as the holding volume increases faster than the surface area. Hence for large capacity applications, instead of increasing the size and volume of a single drum shell, Inoue [74] from Ebara Corporation proposed a similar generator with a regular array of small finned drum shells so as to increase the capacity of the regenerator. For generators that employ a double shell combustion chamber as the heat source, where the refrigerant-rich solution is channelled through the ostensibly annular gap between the two shells, the refractory wall member - that has to be made detachable in order to access and clean the smoke tubes - often suffers from low durability, low reliability, and high manufacturing or maintenance cost. This is due to the relatively large dimensions and the complicated shape that has to be formed with materials having low formability and low strength. Inoue et al. [75] from Ebara Corporation therefore disclosed a high temperature double shell generator which enjoys high durability and reliability by minimizing the dimension and intricacy of the detachable portion of the combustion chamber, while preventing a local solution stagnation point within the modified double shell jacket. The local overheating at the contacting surface of the flame on a finned tube that contains the working solution may result in the generation of incondensable gas, which leads to the deterioration of the chilling performance as well as to corrosion. Hence, Inoue et al. [76] from Ebara Corporation proposed a double shell generator as shown in Fig. (17) which comprises an upstream assembly of staggered bare heat transfer tubes and a downstream assembly of

staggered finned heat transfer tubes, with the provision that the two tube assemblies are aligned instead of being staggered. This prevents the first row of finned tubes from bearing the brunt of the combustion gas, which could lead to overheating given their enhanced heat transfer characteristics. Another invention, on a similar vein, by Machizawa et al. [77] from Hitachi concerned a generator that has a buffer plate between the combustion fuel burner and the heat transfer fins so as to provide a curved flame channel, thereby preventing local overheating. Funaba et al. [78] from Hitachi invented a flue-less high temperature generator with flat instead of circular heat transfer tubes for solution flow, as shown in Fig. (18). It reduces the NOx and CO generation, and results in a compact generator being built with an inexpensive and long-life material. In contrast with Fig. (17), the confined channels between bare flattened tubes serve as a multitude of narrow combustion chambers. There is no concern of local overheating, corrosion and crystallization as the two dimensional solution flow field within the flattened tubes effectively prevent dry-out zones within the tube, and allowed for effective part-load operation. The flame side temperature boundary layers effectively reduces the flame temperature and suppresses NOx level to less than 30ppm, whereas the flame inviscid zones between the boundary layers allows the CO to be effectively oxidized so that CO level is less than 100ppm. Nakamura et al. [79] from Sanyo Electric Corporation disclosed a flue-less double shell high temperature generator with circular heat transfer tubes for solution flow, as shown in Fig. (19). They eliminated the risk of local hot spots within the tubes by sprinkling the refrigerant-rich solution


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1 - Burner, 2 - Combustion chamber, 2 - Flue, 3 - Bare tube assembly, 4 - Finned tube assembly, 5 - Liquid solution space, 6 - Shell, 12 Combustion gas, 31- Bare heat transfer tube, 41 - Finned heat transfer tube Fig. (17). A direct fired generator [76].

1 - Outer shell, 1A - High temperature generator, 2 - Inner shell, 3A - Flat and bare solution tubes in contact with the flame of burner, 3F Flat and finned tube in the inner shell, 6 - Solution inflow tube, 7 - Solution outflow tube, 8 - Refrigerant vapour outflow tube, 10 - Flue box, 11 - Flue, 11A - Combustion chamber, 15 - Burner. Fig. (18). A high temperature generator with flat instead of circular heat transfer tubes [78].

from the top and at the effluent end of the generator. Gravitational effect allows for a heightened convective solution flow as shown by the convective cell in the figure, thereby nullifying the risk of local hot spots within the tubes and allows for a direct contact between the flame and the tubes. Inoue et al. [80] from Ebara Corporation invented an exhaust gas driven high temperature generator in which the

heat transfer tubes conveying refrigerant rich solution are in direct contact with the hot gas, as shown in Fig. (20). Local hot spots are prevented by downcomers which facilitate convective movement of the solution. The downcomers are judiciously positioned away from being in direct contact with the exhaust gas so as to prevent overheating. Baffle plates are incorporated to realize a counter current arrangement to promote heat transfer.



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4 - High temperature generator, 35 - Burner inlet, 39 - Liquid pipe, 41A-B - Double furnace wall, 43 - Combustion gas outlet, 45 - Fins on liquid pipe, 47 - Solution inlet. Fig. (19). A flue-less high temperature generator with gravity assisted convective solution flow within vertical circular heat transfer tubes [79].

71 - Generator, 74 - Vertical heat transfer tube, 75 - Gas-liquid separation chamber, 76 - Solution supply chamber, 78 - Absorption weak solution, 79 - Absorption strong solution, 80 - Refrigerant vapour, 81 - Exhaust gas, 82 - Downcomer, 84 /85 - Baffle plates. Fig. (20). An exhaust gas driven generator [80].

An issue regarding an exhaust gas driven absorption chiller is that during shut down condition, any supply of exhaust gas, even through leakage via dampers, to the generator will trigger solution crystallization and dewing in the duct works, resulting in corrosion. Yamazaki [81] therefore installed a damper at the duct leading to the generator, a damper at the bypass gas duct, and another damper at the exhaust end of the generator to prevent backflow. When the chiller is shutdown, the short section of isolated ductwork connected to the generator is pressurized by a blower to prevent leakage and dewing. To increase the heat transfer efficiency and decrease the gas side pressure drop of an exhaust fired high temperature generator, Jung et al. [82] from UTC Power inserted a series of turbulators into each of the smoke tubes within a high

temperature generator. The turbulators have flanges with different cut-outs that minimize wake at the downstream of the exhaust gas flow, and thus reduce the pressure drop across the exhaust flow path. Such a configuration significantly increases the heat transfer coefficient and reduces the friction factor. 5. IMPROVEMENTS TO AQUEOUS BROMIDE AS AN ABSORBENT

LITHIUM

While aqueous lithium bromide solution, other than aqueous ammonia, is the industrial standard for absorption chillers, there is an ongoing effort in improving its performance. Iizuka et al. [83] from Yazaki Corporation developed an absorbent solution consisting of a mixture of lithium


bromide, lithium iodide and lithium chloride at a ratio of 1:0.1-1:0.05-0.50 that has a sufficiently low crystallization temperature that it could avoid crystallization in the absorbent solution during the operation cycle of the absorption chiller. Kujak [84] from Gas Research Institute developed a refrigerant/absorbent composition, which comprises an aqueous solution lithium halide, zinc halide, and a corrosioninhibiting amount of thiocyanate ion. These compositions are less corrosive and formed less hydrogen without losing their heat transfer effectiveness. 6. ADDITIVES Additives are important ingredients to the lithium bromide solutions. They can be broadly classified into heat and mass transfer additives, crystallization suppressants and corrosion inhibitors. 6.1. Heat and Mass Transfer Additives As described by Bourne et al. [85], the addition of octyl alcohol (2 ethyl-n-hexanol) to aqueous lithium bromide produces a substantial increase in the overall capacity of the refrigeration machine. This is because the presence of octyl alcohol creates a turbulent film on the exterior of the absorber tubes, which enhances heat transfer. In the same vein, Chandler [86, 87] from Gas Research Institute disclosed certain ethers that improved the heat and mass transfer characteristics of the fluid in an absorption chiller, particularly at the high temperature stages of the thermal transfer loops. 6.2. Crystallization Suppressants In order to depress the crystallization temperature of the working fluid without depressing the vapour absorption ability, Shoji et al. [88] from Kawasaki Thermal Engineering Co. Ltd. added caesium chloride in an amount of between 0.01 wt% to 15 wt%, based on the weight of an aqueous 64% lithium bromide solution. Ring et al. [89] added 5-amino2,4,6-trioxo-1,3-perhydrodizine-N,N-diacetic acid and claimed to be able to lower the minimum working temperature of the aqueous lithium bromide to -5°C at concentrations up to 62 wt%. The amount added ranges from 200 molar ppm to 5000 molar ppm and does not have significant effect on the heat capacity of the solution, the solution rheological properties, the solution mass transfer coefficients, or the ability of the solution to absorb water vapour and transfer heat. Ethylene glycol is known to be an anti-crystallization additive to the working fluid. However, Modahl [90] from Gas Research Institute found that ethylene glycol anti-crystallization additive is not sufficiently stable at the typical high generator temperatures to withstand continuous and repeated cycling through the high temperature circuit. 6.3. Corrosion Inhibitors Itoh et al. [91] from Westinghouse Electric Corporation developed an absorbing solution for an absorption chiller comprising lithium bromide solution, 5-150 ppm by weight of nitrates and 0.1-0.4% by weight of LiOH, octyl alcohol,

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and is free from any triazole compounds. This concoction protects the absorption chiller against corrosion and particularly pitting corrosion. Apart from this, Downey [92] from Carrier Corporation invented a corrosion inhibiting working fluid and its manufacturing method. Downey added water-soluble compounds containing molybdates, borates and silicates to aqueous lithium bromide. More recently, Modahl [93] invented an aqueous solution of zinc and lithium bromides with an additional amount of hydroxide ion, which can practically prevent corrosion of the high temperature circuit component in an absorption chiller without raising the freezing point of the solution. 7. NOVEL COOLING

APPLICATIONS

OF

ABSORPTION

Apart from applying absorption chillers to normal industrial and residential cooling, a lot of patents have been filed on cogeneration, cooling power generator, solar energy utilization and other applications. 7.1. Cooling with Low-Grade Cogeneration Systems

or

Waste

Heat:

Cogeneration systems offer the opportunity to reduce electricity consumption for a building complex, factory, hospital or local community and ensure the continuous availability of electrical energy during blackouts or brownouts while simultaneously providing cooling or heating. The system efficiency can be improved where power, heating and thermally actuated refrigeration result from the improved utilization of a single fuel source by independently controlling the simultaneous amount of each in accordance with time-varying user needs. A highly desirable cogeneration system and method was disclosed by Meckler [94, 95]. In this cogeneration system, the waste heat from a fuelled turbine that generated electricity is utilized to produce hot water as well as using the waste heat as the heat source for an absorption chiller to produce cooling. It allows a more efficient use of fuel, and is versatile in producing variable quantities of electrical power, refrigeration and heat according to the demand. Hsu [96] from Ztek Corporation also disclosed a system for combined electricity generation, heating, cooling and ventilation. This invention integrates an electrochemical converter such as a fuel cell for the production of electricity, with a heating, ventilating and cooling system. They disclosed the coupling of a fuel cell to the absorption chiller and also disclosed the interface exchange element for convectively coupling an electrochemical converter to an HVAC system. With the ever more popular application of micro turbines, Dettmer [97] from Elliott Energy Systems Inc. disclosed a direct fired absorption chiller which is combined with a micro turbine engine. The system includes a bypass valve interconnecting the discharge end of the recuperator and the discharge end of the turbine so as to maintain a constant heat load delivered to the chiller. A temperature monitoring sensor actuates the bypass valve to assure that a proper heat load is maintained at the chiller.


Rumsy [98] invented a heat recovery cooling system that employs absorption chillers powered by waste heat generated by electronic equipment such as computers in a computer server room where excess heat is detrimental, to provide cooling to ensure no overheating to the electrical equipment. Longardner [99] invented a system that improves the efficiency of a step-up or step-down power transformer used in the transportation of electricity in the electrical power generation industry. This is achieved by integrating an absorption chiller into the system to use the heat dissipated from the transformer to produce chilled water that is recirculated to lower the temperature of the transformer and improve its performance. 7.2. Cooling Power Generator Inlet Air It is well known that the efficiency of a power generator increases with decreasing inlet gas or air temperatures. Some researchers harness the waste heat from a power generator to drive an absorption chiller which in turn cools the inlet air or gas to the generator. This configuration can substantially increase the efficiency of the power plants. Polizzotto [100] from Union Carbide Corporation utilized the steam produced by a cogeneration facility to drive an absorption chiller; the chilled water so produced is then used to cool the gas to the turbine. Lehto [101] exploited the effluent gas from a turbine generator to drive absorption chillers and a liquid desiccator. The chilled water from the chiller is then used for evaporative cooling of the incoming air to the turbine. Powered also by the chilled water, the liquid desiccator is positioned downstream of the evaporative cooler to remove the introduced moisture prior to the cooled air entering the turbine. 7.3. Solar Energy Leonard [102] from Carrier Corporation disclosed an absorption chiller that is mainly powered by solar energy with the aid of a highly efficient electrically driven chiller in 1978. However this kind of conventional system has the inconveniences of having a large solar collecting unit and being very expensive. Later Tanaka et al. [103] from Osaka Gas proposed a solar driven absorption chiller that uses an absorption cooling system and a compression cooling system. The solar hot water is used to power the absorption cooling system, and an electrical supply is needed for the compression cooling system. Apart from the inherent benefits of an absorption/compression cooling system, this system enjoys a better utilization of the obtained solar heat energy with a lower equipment cost, and a shorter payback period. Bellac et al. [104] from York Research Corporation disclosed a turbine generator, which incorporates a solar powered absorption chiller that cools the inlet air to the generator so as to increase power generation. 7.4. Other Applications MacCracken [105] disclosed a method for air conditioning a building that has a lesser night time load than a daytime cooling load. The method consists of a chiller that is operated around the clock, which is powered by an electric generator driven by a fuel fired engine, and an absorption


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chiller for freezing ice to store in an ice bank by using the heat generated from the fuel fired engine. The ice melts in the daytime to act as a supplement to the electric chiller and absorption chiller in cooling the building, with an aim to lower the peak cooling demand and reduce the overall capacity of the cooling equipment. A few patents combine an absorption cooling system with a liquid desiccant dehumidification subsystem. Wilkinson [106] from Gas Research Institute described a hybrid air conditioning system in which the absorption refrigeration system is used to handle the sensible heat load of a building while the liquid desiccant dehumidification system is used to handle the latent heat. Later, Maeda [107] from Ebara Corporation disclosed a desiccant assisted air conditioning apparatus, which incorporates an absorption heat pump device. The cooling derived from the absorption heat pump is used to cool the process air and the heat from absorption heat pump is used to regenerate the desiccant. The utilization of heat rejected by the heat pump results in significant energy conservation and high system efficiency. 8. CURRENT & FUTURE DEVELOPMENTS Absorption cooling technology has been extensively researched and many patents have been filed worldwide which lead to substantial improvements to system performance and reliability. We expect the following areas of improvement to be targeted in the future. (1) Improve the system conversion efficiency, reliability and compactness without adding complexity into the design and operation of system. (2) Lower the heat source temperature so as to enhance the energy recovery from low grade waste heat. (3) Minimize corrosion to reduce the initial and operating cost of chiller system. (4) Extend the application of absorption cycles in renewable energy applications. The following aspects are highlighted. 8.1. Working Fluids Every aspect of absorption cooling technology is governed by the properties of working fluids. Hence the advent of new working fluids would enable completely different cycles than those discussed in this study, and the design of system components would also be changed significantly. 8.2. Low Temperature Heat Input The high COP value achievable by a triple effect chiller is 1.6 [108], and in general a triple effect system will need to have a direct-fired generator where natural gas or other fossil fuels is consumed. However using direct-fired absorption cooling systems instead of vapour compression systems are not energy efficient. Considering an electrical power plant having an efficiency of 60%, and the COP of a centrifugal chiller being greater than 6, the combined system COP is 3.6, which is far greater than 1.6 of a triple effect absorption chiller. Given such a background, low temperature waste heat driven absorption cooling system should be pursued. In addition, waste heat abounds at around 60 to 90oC, which mainly hails from process plants, cogeneration units, fuel cells, and computers. The vision of integrated energy use is


an important research direction to achieve higher energy efficiency and smaller carbon footprint.

Wang and Chua [13] [14] [15]

8.3. Geothermal Application Geothermal energy is a valuable energy source for absorption cooling systems, and it can be readily exploited in cities located on deep sedimentary basins. Australia is one of the countries endowed with rich geothermal energy resources. In the Perth Basin of Western Australia, it is estimated that 90°C groundwater in aquifers with good permeabilities is available at a depth of 2-3 km. This is sufficient to drive single stage absorption chillers and make geothermal air conditioning viable. In recent decades, many patents have been disclosed claiming improved COP and reliability of absorption technologies and promoting the application of absorption cooling. However, most claims of high efficiency and reliability in patents cannot be independently verified and in particular demonstrated with real machines. Some of the innovative systems show promise and should be demonstrated experimentally in the public domain so that the flaws preventing them from widespread commercialization can be corrected. Reducing the cost of the system by developing modified working fluids and additives to inhibit corrosion, inventing new absorption cycles and higher performance absorbers and generators, as well as new applications of absorption technologies are still major research areas for absorption technologies. Finally, more demonstration pilot plants should be supported for verifying the viability of new technologies.

[16] [17]

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

ACKNOWLEDGEMENTS

[38]

We gratefully acknowledge the assistance of Christopher Wen Siong Tan for performing a comprehensive patent search and the support of the Western Australia Geothermal Centre of Excellence. We also gratefully thank Dr. Frank Horowitz for kindly proofreading our manuscript.

[39]

[40] [41] [42]

CONFLICT OF INTEREST There is no conflict of interest. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

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Absorption Cooling: A Review of Lithium Bromide-Water Chiller Technologies

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